The present invention relates generally to optical transmission of signals and more particularly to an optical communication network of the kind having a multimode optical fiber that receives a multiple mode beam of light from a vertical-cavity, surface-emitting laser being operated in multiple modes or multiple filamentation.
Optical communication systems are used to carry information from one location to another. One of the advantages of optical systems is that they have extremely wide bandwidths. This means that optical systems can carry much more information than can other kinds of communication systems such as radio or microwave. For example, nearly all long-distance telephone calls are carried by optical communication systems because a single optical fiber can carry thousands of conversations at the same time. Optical systems also offer the potential of carrying large quantities of digital data for high-speed computers more efficiently and economically than other communication systems.
Every optical communication system includes, at a minimum, three elements: a transmitter that generates a beam of light and modulates the beam with data to be transmitted, a receiver that receives the beam of light and recovers the data from it, and a medium such as an optical fiber that carries the beam of light from the transmitter to the receiver. Typically the transmitter uses a laser or a light-emitting diode ("LED") to generate the light beam. The receiver uses photodetectors or the like to receive the beam. The medium may be an optical waveguide or the like instead of an optical fiber.
Light may travel through an optical medium in single mode or multiple modes. In general, a "mode" of an electromagnetic wave can be defined as a stationary pattern of the wave. In the special case of a beam of light (which may be thought of as an electromagnetic wave in the optical portion of the spectrum), a mode is a wave pattern that does not change the shape of its transverse field distribution as it propagates through the medium.
A given optical medium may be capable of supporting many modes or only a single mode. This is determined by physical parameters such as--in the case of an optical fiber--the diameter of the fiber and the difference between the indices of refraction of the core and the cladding.
Likewise, many lasers can be caused to operate in single mode or in multiple modes. This can be done by a suitable choice of device structure and drive conditions. Multiple mode operation has generally been understood to consist of multiple modes in one laser cavity. However, studies have shown that multiple mode laser operation can occur with filamentation due to non-uniform gain or loss. This is especially true for lasers with large transverse dimensions compared with the wavelength. For convenience, the terms "multiple mode" and "multimode" as used herein to describe the operation of a laser will include both multiple modes in a single laser cavity and multiple filamentation.
Optical communication systems are subject to various kinds of losses and limitations. Among these are intermodal dispersion, chromatic dispersion and mode selective losses. All of these have the effect of decreasing the signal-to-noise ratio, and therefore it is desirable to eliminate or minimize them as much as possible.
Intermodal dispersion becomes worse as the length of the fiber increases. Intermodal dispersion only affects multimode fibers, and therefore single mode fibers are preferred for communication over long distances. As used herein, a "long" distance means a distance that is more than a few hundred meters and a "short" distance is one that is less than a few hundred meters. Of course, it should be understood that this is an approximation; multimode fibers up to a few kilometers in length have been used successfully, but usually when the required length of the fiber exceeds a couple of hundred meters a single mode fiber will be used.
Chromatic dispersion also becomes more severe as the length of the fiber increases but, unlike intermodal dispersion, chromatic dispersion affects both single mode and multimode fibers. The adverse effects of chromatic dispersion can be minimized by using a highly coherent laser because such a laser produces a light beam of very narrow spectral width. Accordingly, highly coherent lasers have been preferred for most optical communication systems, especially for communication over long distances.
Of course, single mode optical fibers can also be used over short distances (less than a few hundred meters), for example to carry digital data from one computer to another in a local network or even to carry data between points less than a meter apart within a single computer. However, multimode optical fibers are preferred for short-distance optical communication systems because their relative ease of packaging and alignment makes them considerably less expensive than single mode fibers.
A drawback of multimode optical media has been that these media are subject to mode selective losses. A mode selective loss may be characterized as a physical condition that affects the optical characteristics of the medium. These losses may be, for example, splices in the medium, power splitters and other devices that are connected to the medium, and physical defects such as poor quality connections and misalignment of components. Although such physical conditions can be reduced by careful design and construction, in practice it is rarely possible to produce a system that is totally free of them. Therefore, all practically realizable multimode optical communication systems will be subject to at least some mode selective losses.
The actual mechanism by which physical discontinuities produce mode selective losses will now be briefly discussed. Interference between different modes in a multimode medium carrying a coherent light beam produces a speckle pattern. Ideally this speckle pattern would remain stationary, but in practice it moves about within the medium. Speckle pattern movement may be caused by physical jostling or other movement of the fiber itself (relatively slow movement) or by laser mode partitioning and the like (relatively fast movement). Movement of the speckle pattern in a system having mode selective losses results in power variations in the received signal. These variations are caused by the mode selective losses and result in a degradation of the signal-to-noise ratio. In digital systems, a degradation of the signal-to-noise ratio manifests itself as an increased bit error rate.
Mode selective losses are described in more detail in such references as Epsworth, R. E., "The Phenomenon of Modal Noise in Analogue and Digital Optical Fibre Systems", Proceedings of the 4th European Conference on Optical Communications, Genoa, September, 1978, pp. 492-501, and in Kanada, T., "Evaluation of Modal Noise in Multimode Fiber-Optic Systems", IEEE Journal of Lightwave Technology, 1984, LT-2, pp. 11-18.
Mode selective losses can be avoided by using a relatively low-coherence light source such as an LED or a self-pulsating laser diode ("SPLD") rather than a highly coherent laser. The use of LEDs in optical communication systems is described in Soderstrom, R., et al., "Low Cost High Performance Components of Computer Optical Data Links", Proceedings of the IEEE Laser and Electrooptics Society Meeting, Orlando, Fla. 1989. A disadvantage of using LEDs in optical communication systems is that the coupling efficiency between an LED and an optical fiber is very low. In addition, LEDs are inherently slow, which limits the maximum data rate.
SPLDs have been used in such systems as the Hewlett-Packard HOLC-0266 Mbaud Fiber Channel multimode fiber data link, manufactured by the assignee hereof; this is described in Bates, R. J. S., "Multimode Waveguide Computer Data Links with Self-Pulsating Laser Diodes", Proceedings of the International Topical Meeting on Optical Computing, Kobe, Japan, April, 1990, pp. 89-90. The coupling efficiency between an SPLD and an optical fiber is better than that between an LED and an optical fiber, but still is not optimal. In addition, the maximum data rate that can be achieved with an SPLD is limited. Neither SPLD nor LED systems have been able to achieve reliable data rates as high as 1 gigabit per second
From the foregoing it will be apparent that there remains a need for a reliable and economical way to carry data at rates exceeding one gigabit per second by means of optical communication systems operating over short distances.